The long-term goal of this research is to understand the structural and functional changes in the hypothalamus that are responsible for the reversible suppression of ovarian function, such as occurs prior to puberty. It is well recognized that increases in response to estradiol (E2) negative feedback play an important role in producing these periods of infertility (e.g., gonadostat theory of puberty). Using seasonal breeding as a model, we have identified a neural circuit (E2 responsive-neurons in the ventromedial POA [vmPOA] and the retrochiasmatic area stimulate A15 dopamine neurons to inhibit GnRH secretion) that is activated to induce the non-breeding (anestrous) season. We have recently developed strong evidence that glutamate and ?-amino butyric acid (GABA) are important controllers of A15 neural activity in anestrus. The experiments in Aim 1 will identify the GABA and glutamatergic neurons afferent to the A15 and test two specific hypotheses to account for seasonal alterations in E2 negative feedback: 1) there is a decrease in E2 receptors in GABAergic neurons during the breeding season, and 2) there is a decrease in input from E2-responsive glutamatergic afferents to the A15 during the breeding season. Studies in Aim 2 will test two alternate hypotheses to account for A15 inhibition of GnRH secretion in anestrus: 1) A15 neurons project directly to GnRH terminals in the median eminence, or 2) A15 neurons stimulate gonadotropin-inhibitory hormone (GnIH) neurons or inhibit a set of kisspeptin-containing neurons to suppress GnRH. Aim 3 is based on evidence that thyroid hormone (T4) actions in the premammillary region and vmPOA are required for the structural changes that lead to activation of this circuit at the transition ot anestrus. Specifically, we will test the hypothesis that brain-derived neurotrophic factor mediates the action of T4 during the transition to anestrus and begin to identify the downstream targets of T4 by determining if it is necessary for seasonal changes in the number of glutamatergic synapses on to A15 neurons. We will use a combination of molecular, anatomical, pharmacological, and physiological approaches to test specific hypotheses relevant to each aim. The results of these studies will provide fundamental information on how the brain changes to shut down, and restart, fertile ovarian cycles. This information may provide novel treatments for pathological conditions, such as precocious puberty or hypothalamic ammenorrhea, in which these changes occur inappropriately. The may also provide insight into the converse situation of insufficient activity of inhibitory neural systems that appears to contribute to polycystic ovarian syndrome.